Simple Nature - Light and Matter

n / A fluxgate compass.is externally applied, the magnetization increases, 2, but eventuallybecomes saturated, 3, so that higher fields do not result in anyfurther magnetization, 4. The external field can then be reduced,5, and even eliminated completely, but the material will retain itsmagnetization. It is a permanent magnet. To eliminate its magnetizationcompletely, a substantial field must be applied in the oppositedirection. If this reversed field is made stronger, then the substancewill eventually become magnetized just as strongly in the oppositedirection. Since the hysteresis curve is nonlinear, and is not a function(it has more than one value of M for a particular value of B), aferromagnetic material does not have a single, well-defined value ofthe permeability µ; a value like 4,000 for transformer iron representssome kind of a rough average.The fluxgate compass example 26The fluxgate compass is a type of magnetic compass withoutmoving parts, commonly used on ships and aircraft. An AC currentis applied in a coil wound around a ferromagnetic core, drivingthe core repeatedly around a hysteresis loop. Because thehysteresis curve is highly nonlinear, the addition of an externalfield such as the Earth’s alters the core’s behavior. Suppose, forexample, that the axis of the coil is aligned with the magneticnorth-south. The core will reach saturation more quickly whenthe coil’s field is in the same direction as the Earth’s, but will notsaturate as early in the next half-cycle, when the two fields arein opposite directions. With the use of multiple coils, the componentsof the Earth’s field can be measured along two or threeaxes, permitting the compass’s orientation to be determined intwo or (for aircraft) three dimensions.11.7.4 Maxwell’s equations in differential formThis chapter is summarized on page 960. Notation and terminologyare tabulated on pages 941-942.716 Chapter 11 Electromagnetism